Heat treatment of columbium and molybdenum base alloys



United States Patent Ofifice 3,297,496 Patented Jan. 10, 1967 3,297,496 HEAT TREATMENT OF COLUMBIUM AND MOLYBDENUM BASE ALLOYS Winston H. Chang and Robert G. Frank, Cincinnati, Ohio,

assignors to the United States of America as represented by the Secretary of the United States Air Force No Drawing. Filed June 7, 1963, Ser. No. 286,430 8 Claims. (Cl. 148-115) This invention concerns the heat treatment of columbium and molybdenum base alloys as a means for improving the useful characteristics thereof.

This invention increases the stress-rupture strength of columbium and molybdenum base alloys by the heat treatment thereof.

The objects of this invention are to increase the rupture strength of the alloys that are described herein. This increase in rupture strength is attributed to (1) the redistribution of the precipitated phases in the grain boundaries and the matrix, (2) the increase in the grain size, and (3) in the case of the F45 alloy of the composition 84% Cb or Nb, 1% Zr, and W, the polygonization of the structure. Where needed, the term polygonization is defined at page 583 of the Encyclopaedic Dictionary of Physics, by J. Thewlis, published in 1962 by Pergamon Press, New York city. Briefly polygonization is where the curved lattice row is turned into a segmented line or a polygon, on heating. Complex polygonization takes place under creep conditions in polycrystalline specimens that are heated after deformation.

By way of example, the invention consists of heating and annealing a semiprocessed alloy of columbium at high temperatures for a sufficient length of time to allow desired microstructural changes to occur in the alloy. The temperature treatment of the alloy is generally above the temperature of the usual processing and heat treating practice for this class of alloys.

The alloys treated in the manner that is disclosed herein have a high-temperature strength that is many times higher than the strength of the same alloy that has been heat treated in accordance with the prior known methods.

The high temperature annealing of a previously worked alloy of columbium and molybdenum, which constitutes the process of the present invention, causes the recrystallization, the grain growth and the grain boundary carbide distribution, when carbon in excess of 0.01% by weight is in the alloy.

Grain growth reduces the amount of grain-boundary areas along which plastic deformation takes place, when a polycrystalline metallic material is stressed at high temperatures, with or without low rates of straining. For a given grain size, the grain-boundary deformation is further restricted by the dispersion of carbides along the grain boundaries. High temperatures increase both the grain growth and the dispersion of carbides along the grain boundaries.

It is on this theory that high temperature annealing is applied to the columbium and molybdenum base alloys here of interest for enhancing the strength of the alloys through the mechanisms of promoting improved grain growth and carbide dispersion along the grain boundaries.

The present invention of heat treatment is applicable to alloys in general and is particularly applicable to columbium base and molybdenum base alloys.

The alloy to be treated is partially worked to break down its ingot structure, by a currently used method selected from the group of extrusion, forging rolling, and swaging.

The high temperature annealing processing of the alloys that are contemplated hereby, is applied to the partially worked alloy and is followed either with or without a final or finishing working operation.

The alloy in its partially worked condition is believed to be necessary in providing the driving force that ac complishes the needed recrystallization and grain growth during the annealing operation of the alloy. The an nealed alloy is used as such or is given a final working, depending on the strength that it is to exhibit in its intended use.

If the alloy is to be used in the work-hardened condition, it is essential that the annealing of the alloy be applied at a stage that is not earlier than before the final working.

The annealing temperature may be anywhere between 2500 F. and 100 F. below the incipient melting temperature of the particular alloy that is being treated. The preferred annealing temperature is determined by such factors as the composition of the alloy, the severity of the plastic deformation of the alloy at the annealing temperature, the extent of the grain growth that is desired and the like. The annealing operation is conducted within a vacuum that illustratively is about 10'- mm. Hg, or within an atmosphere that inhibits the oxidation of the alloy and the undesirable contamination of the alloy being treated.

It will be noted that the heat treatment that is disclosed herein as being applicable to alloys of columbium and molybdenum also is applicable to alloys of the additional refractory metals tungsten, tantalum, and chromium.

Example 1 The potency of the high temperature annealing that is contemplated hereby is represented illustratively by the alloy that has the composition by weight of 99.7% molybdenum, 0.15% zirconium, and 0.15% carbon.

This alloy was arc-melted into an ingot. The ingot was extruded at the temperature 2820 F., which is about 1550 C., as a bar and the bar was rolled at the temperature 2640 F. or about 1450 C.

One section of the bar was swaged at 2300 F. or 1260 C. Another section of the bar as rolled was annealed at 4300 F. or 2371 C. for six hours in an induction-heated vacuum furnace with the vacuum maintained at 0.1 to 0.2 mw. or within the range of from 10 to 10 mm. Hg. The annealed bar was then also swaged at 2300 or 1260 C. to the same reduced diameter as was the first section.

Both of the swaged bars of corresponding diameters, one annealed for six hours at 4300 F. and the other not annealed, were stress relieved in hydrogen at 2400 F. or about 1316 C., for one hour and then they were machined into specimens for tensile tests within the vacuum of between 10- and 10 mm. Hg and were rupture tested in argon. The rupture tests provided the test results:

2,000 F. Tensile 2,200 F./30.000 p.s.i. Rupture Condition Ult., p.s.i.

Elong, percent Life, Hrs.

Elong., percent Swaged alone 16. 5 4,300" F. annealed and swaged Example 2 During the evaluation of Phase III Cb alloys, it was found that high temperature, vacuum heat treatments result in the precipitated phases going into solution and redistributing themselves in the structure in a manner that results in the increased hardness of the resulting product at room temperature.

The alloys processed are represented by the alloy F41 that consists of by weight 88.35% Cb, 10% Mo, 1% Ti, 0.5% Zr and 0.15% C; and the alloy F45 that consists of by weight 84% Cb, 15% W, and 1% Zr.

The alloys F41 and F45 were heat treated in a vacuum of about 10 mm. to 10 mm. Hg at 3500 F. for one hour and then were removed from the furnace and returned to room temperature.

The alloys F41 and F45 are placed in crucibles that are chemically non-reactive with the alloys. The crucibles that contain the alloys are placed in an arcfurnace and the alloys are melted and are cast into ingots. The ingot of the alloy F41 is heated to 1500 C. and is extruded as a rod. The F45 alloy ingot is heated to 1600 C. and is extruded as a rod. 1500 C. is 2732 F. 1600 C. is 2912" F.

Both the F41 and the F45 extruded pieces are annealed within the vacuum for six hours at 2200 R, which is 1204 C.

The extruded pieces are maintained within the temperature range of 2350 to 2000 F. as they are swaged to a little less than one-half of their previous diameters. 2350 F. is 1288 C. and 2000 F. is 1093 C.

The swaged specimens of F41 and F45 alloys were heat treated for one hour at 3500 F. or 1927 C. in a vacuum of to 10- mm. Hg.

The resultant products provided an F41 alloy that, when it was subjected to a stress range in the order of 20,000 to 25,000 p.s.i. increased its life hours from 18 to above 206, and an F45 alloy that was subjected to a stress range in the order of from 22,500 to 30,000 p.s.i. increased its life hours from 100 to above 175.

It is to be understood that the alloys and their processing that are cited herein are submitted as experimentally confirmed fiindings that can be duplicated successfully under the same conditions and that limited variations may be made in the alloys and in the procedures without departing from the spirit and scope of the present invention.

We claim:

1. A method of heat treating an alloy selected from the group consisting of a columbium base alloy and a molybdenum base alloy for the purpose of improving the useful characteristics, and particularly the strength thereof, consisting of the steps of (1) partially working the alloy and thereafter (2) annealing the alloy within the temperature range of (a) about 2500 F. to (b) about 100 F. below the incipient melting temperature of the alloy Within a vacuum of about 10 mm. Hg for about one to six hours.

2. The method of claim 1 in which the step of partially working consists of an extrusion step.

3. The method of claim 1 in which the partially working step consists of a forging step.

4. The method of claim 1 in which the step of partially working consists of a rolling step.

5. The method of claim 1 in which the step of partially working consists of a swaging step.

6. The heat treatment defined by the above claim 1 applied to an alloy that by weight consists of 99.7% Mo, 0.15% Zr, and 0.15% C.

7. The heat treatment that is defined by the above claim 1 applied to an alloy that consists of by weight 88.35% Cb, 10% Mo, 1% Ti, 0.5% Zr, and 0.15% C.

8. The heat treatment that is defined by the above claim 1 applied to an alloy that consists by weight of 84% Ch, 15% W, and 1% Zr.

References Cited by the Examiner UNITED STATES PATENTS 5/1962 Redden 14811.5 7/1965 Chang 174 OTHER REFERENCES Transactions of The Metallurgical Society of AIME, vol. 215, December 1959, pages 898-901. 

1. A METHOD OF HEAT TREATING AN ALLOY SELECTED FROM THE GROUP CONSISTING OF A COLUMBIUM BASE ALLOY AND A MOLYBDEUM BASE ALLOY FOR THE PURPOSE OF IMPROVING THE USEFUL CHARACTERISTICS, AND PARTICULARLY THE STRENGTH THEREOF, CONSISTING OF THE STEPS OF (1) PARTIALLY WORKING THE ALLOY AND THEREAFTER (2) ANNEALING THE ALLOY WITHIN THE TEMPERATURE RANGE OF (A) ABOUT 250$F. TO (B) ABOUT 100*F. BELOW THE INCIPIENT MELTING TEMPERATURE OF THE ALLOY WITHIN A VACUUM OF ABOUT 10**-5 MM. HG FOR ABOUT ONE TO SIX HOURS. 